Sophie
Harrington
Gardens in the sky
Growing plants in space
Thinking of space science conjures up images
of planets, stars, and sleek spaceships cruising
through the galaxy. But humans aren’t the only
life forms we’re sending into space. As Sophie
Harrington explains, some of the most cutting
edge plant-science research is being carried out
hundreds of kilometres above our heads.
Missing gravity
The presence of gravity is a given for life on
Earth. It’s no surprise that many organisms use
this ever-present force to guide their growth and
development. Plant growth is highly directed by
gravity, as roots grow downwards while shoots
reach up towards the sky, in the opposite direction.
These are known respectively as positive gravitropism
and negative gravitropism. This response to gravity is
crucial in directing plant growth down on Earth,
but up in the International Space Station (ISS) it’s
another story.
Key words
plants
tropisms
space travel
Mars
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Growing up in space
ESA
Any plans to send humans into space for long
periods of time will require the ability to grow plants
to supplement the astronauts’ diet. However, until
recently, scientists weren’t sure how microgravity
might affect the life cycles of plants. It’s not much
good sending all sorts of seeds into space if they
aren’t going to grow properly.
Arabidopsis thaliana, a small garden weed, is the
favoured plant for many scientists in part due to its
short lifecycle. This makes it particularly suited for
studying how life in space might affect its growth.
Luckily for the astronauts, the little plants were
able to grow quite normally in space. In fact, not
only did their leaves, flowers, and fruit develop
normally, but the leaves remained dark green for
longer than they would on Earth. This suggests
that microgravity may somehow influence leaf
senescence, slowing plant ageing. Scientists still
don’t know what’s behind this peculiar result, or if
it also occurs in other plants. Even more intriguing
is the question of whether humans and animals
might also benefit from a visit to space. Perhaps
the fountain of youth is actually in outer space.
NASA
In orbit over 300 kilometres above the Earth, the
ISS experiences only microgravity. The gravitational
force from the Earth holds the station and
its contents in orbit, so that everything inside
experiences zero gravity (sometimes known as
microgravity). But if gravity is so important, how
do plants manage to grow properly on the ISS?
It turns out that plant responses to light, known
as phototropism, may be just as important as gravity
in determining how plants grow. Plant responses
to light go in the opposite direction to gravity,
with shoots growing towards the light (positive
phototropism), while roots grow away (negative
phototropism). It appears that on Earth both light
and gravity play a role in directing plant growth.
This also explains why, on Earth when seeds are
germinated in a dark room, the shoot still grows up
and the roots down. In that case, gravity is sufficient
to determine the direction of plant growth.
Intriguingly, studies on the ISS suggest that the
response of plants to different wavelengths of light
may change in microgravity. Red light was found to
promote plant growth on the ISS, while it doesn’t
back on Earth. It may be that the red-light response
is suppressed on Earth by gravity, only appearing
when plants don’t perceive any gravitational force.
This result was a big surprise to scientists who had
originally thought that only blue light could direct
plant response to light.
By studying plants in microgravity, researchers
are able to remove gravity from the equation.
Hopefully this will lead to even more discoveries
about how light and other environmental cues
interact with gravity (or the lack of it) to influence
plant growth both on Earth and in space.
Plants growing on the ISS need to use as little space
as possible. Here Arabidopsis seedlings are growing in
a small dish, testing the effect of microgravity on root
and shoot growth.
The International Space Station in orbit.
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The next step in growing plants sustainably in space
is making sure they can produce seeds in space, and
that these ‘space seeds’ can in turn germinate and
grow themselves. A study by NASA attempted to
grow Arabidopsis from seed to seed on the ISS. When
the second generation seeds were sent back to Earth
and germinated, the plants didn’t show any visible
abnormalities, boding well for the future.
Further work by Russian cosmonauts with pea
plants showed that four generations of the plants
could be grown on the ISS without any significant
changes from the original. This suggests that
microgravity and other space conditions do not
affect the reproductive ability of at least some plants.
It’s looking more and more likely that astronauts
will one day be able to significantly supplement their
diet with home-grown vegetables and fruit.
Sending humans to Mars is a big operation,
with public agencies like NASA and the
European Space Agency as well as private
groups such as Mars One working to make
it a reality. But sending humans to Mars is
going to need more than just a big rocket.
Feeding new colonists on the red planet will
likely rely on easily-assembled greenhouses
shipped with the astronauts to Mars. In these,
colonists will be able to grow vegetables such
as tomatoes and peppers to supplement
their long-lasting, pre-packaged space food.
Besides tasting good, these plants will provide
needed nutrients to the colonists.
NASA
A green future for Martian
exploration
In early plant experiments on the Mir space station,
Russian cosmonauts found that their favourite time of
day was tending to the plants. Gardening helped the
astronauts unwind, with many becoming very attached
to their plants. More recently, NASA astronaut Don
Pettit (shown here) even started writing a blog from
the point of view of the courgette plant he was tending
to. That’s dedication!
Future colonists on Mars will cultivate a variety of
vegetables and fruits to supplement their diet. It
will be a long time, though, before colonists can
become self-sufficient. Early missions are likely to
depend on biennial deliveries from Earth.
Kevin Gill
Most plans for colonies on Mars expect to use
hydroponic systems to create their gardens.
Some scientists, however, are exploring the
ability of plants to grow on Martian soil. This
would be crucial for an expanding colony,
reducing the need for increasing greenhouse
capacities. In fact, a recent paper suggested
that many plants grow even better on Martian
soil than they do on Earth soil. Perhaps the
Red planet could one day be even greener
than our own.
An artist’s interpretation of how a fully terraformed
Mars might look
We know plants can grow in space, but what about
on other planets? Researchers have been doing
work simulating Martian and lunar conditions on
Earth (see Box on left), but it’s hard to study the
Moon when we’re stuck on Earth.
Luckily for us, NASA is about to send a miniature
greenhouse to the Moon, filled with Arabidopsis,
basil and turnip seeds. If all goes to plan, in late
2015 these will be the first plants to grow on a
different world. Researchers will follow the growth
of the plants for 5-10 days, comparing their growth
to that of the Earth-based controls.
NASA
NASA Pat Rawlings
One small step for a plant
The Lunar Growth Module will hitchhike to the Moon
on a rocket sent up by a private space company.
By sending these seeds to the Moon, scientists
hope to discover whether plants would be able
to germinate and respond appropriately with the
different environmental signals on the Moon.
While the first experiment will focus mainly on
germination and phototropism, if all goes to plan
more experiments will be flown to the Moon.
As plant growth is studied for longer periods,
the effect growing on the Moon has on sexual
reproduction, dominant and recessive traits, and
multi-generational growth amongst others can be
studied.
Who knows – it might not be too long before all
sorts of plants are growing on different worlds and
in outer space.
Sophie Harrington is a final-year undergraduate at the
University of Cambridge studying plant sciences
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